A conventional method for measuring transmission parameters of a pair type cable composed of balanced pairs comprises the steps of connecting a measuring equipment to a near-end of a balanced pair, while opening a far-end of the balanced pair, and measuring a far-end-open input admittance Yf or impedance Zif of the balanced pair. The conventional method for measuring transmission parameters of a pair type cable composed of balanced pairs further comprises the steps of short-circuiting a far-end of the balanced pair, and measuring a far-end-short input impedance Zs (Zis) of the balanced pair.
The measured values are substituted into the equations (1) and (2) or (1') and (2'). ##EQU1## where Zb is a characteristic impedance, .gamma.b is a propagation constant (.gamma.=.alpha.+j.beta., where .alpha. is an attenuation constant, and .beta. is a phase constant), and L is a length between the near-and far-ends of the balanced pair. ##EQU2## where Zo is a characteristic impedance, .gamma. is a propagation constant (.gamma.=.alpha.+j.beta., where a is an attenuation constant, and .beta. is a phase constant), and L is a length between the near-and far-ends of the balanced pair.
In the conventional method for measuring transmission parameters of a pair type cable composed of balanced pairs, a balanced type equipment is used.
These days, the transmission rate of digital signals transmitted through pair type cables has become high, wherein transmission frequencies of the digital signals are at a high frequency band. Therefore, the transmission parameters are required to be measured at the high frequency band.
In the conventional method for measuring transmission parameters of a pair type cable composed of balanced pairs, however, there is a disadvantage in that a balance type equipment having a good balance degree at a high frequency band is difficult to be commercially manufactured. Actually, such balanced type equipments have not been manufactured, but unbalanced type equipments have been widely used at a high frequency band for the purpose other than a method for measuring high frequency parameters of a balanced pair.